This invention relates to a positive photoresist composition comprising a polymer in the form of a novolac resin in which some hydroxyl group hydrogens are substituted with 1,2-naphthoquinonediazidosulfonyl groups.
The recent down-sizing trend of electronic equipment promotes the development of LSIs with higher integration, for which multi-pin thin layer bonding is widely employed. In the multi-pin structure, bumps are requisite as connection terminals. Photoresist materials are employed in forming bumps on LSIs by a plating process.
As a result of an increased number of pins and a reduced pin spacing toward higher density on LSI, the shape of bumps formed using thick film resist is required to be perpendicular (or have straight sidewalls). The photoresist for use in the plating process has to meet requirements including thick-film performance, high sensitivity, shape perpendicularity, high resolution, plating resistance, and complete stripping.
However, prior art resist materials are difficult to meet the above requirements. Several problems arise when a positive photoresist comprising a novolac resin and 1,2-naphthoquinonediazidosulfonate as used in the conventional LSI manufacture is used in the thick film technology without modification. An increase in film thickness increases the relative amount of naphthoquinonediazido to enhance absorption, incurring a noticeable lowering of sensitivity. Also, on account of the light absorption of a photosensitive agent, a difference in exposure energy occurs between the surface layer and the bottom of resist, exacerbating the shape and resolution (aspect ratio). In addition, the resist will crack because it cannot withstand the stress of metal growth during the plating.
To overcome these drawbacks, a resist having added an acrylate copolymer having flexibility was developed (see Surface Technology, vol. 44, page 490, 1993). To comply with electronic devices of higher density, resists are required to have a higher resolution. With respect to the in-plane film thickness distribution, there is a concern about the degradation of uniformity in a coating plane as a result of thick film. There is an increasing demand for the uniformity of in-plane film thickness distribution. It is also expected that the thick film gives rise to problems like a lowering of throughput in the device manufacturing process and a shortage of developing power of prior art developers. However, it was difficult to develop a photoresist that satisfies all of the foregoing requirements.
An object of the invention is to provide a positive photoresist composition which when used in thick film pattern formation, has the advantages of uniformity, high sensitivity, high resolution, good pattern shape, substrate adhesion, film retention, shelf stability, high throughput and resistance to metal growth stress.
It has been found that the above-mentioned problems are solved by using an alkali-soluble cellulose whose glucose ring substituent groups are substituted with organic groups at a specific rate of reaction, as a binder in a photoresist composition.
According to the invention, there is provided a positive photoresist composition comprising (A) a polymer and (B) an alkali-soluble cellulose resin. The polymer (A) is in the form of a novolac resin comprising recurring units of the structure represented by the general formula (1): 
wherein m is an integer of 1 to 3, and having a weight average molecular weight of 1,000 to 30,000 based on polystyrene standards and in which 3 to 27 mol % of the hydroxyl group hydrogens on the novolac resin are substituted with 1,2-naphthoquinonediazidosulfonyl groups. The alkali-soluble cellulose resin (B) is represented by the general formula (2): 
wherein R is independently a hydrogen atom, an alkyl group of 1 to 6 carbon atoms, a hydroxyalkyl group of 1 to 6 carbon atoms, an acyl group of 2 to 15 carbon atoms or an organic group of the structural formula (3): 
wherein R1 is an alkylene group of 1 to 6 carbon atoms, phenylene group or cyclohexylene group, in a range providing an acid value of 30 to 150, the proportion of the organic group of formula (3) in R being on average 2 to 30 mol % per unit glucose ring, and n is an integer of 2 to 10,000.